The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.
Exploring, drilling, completing, and operating hydrocarbon and other wells are generally complicated, time consuming, and ultimately very expensive endeavors. Thus, in order to maximize hydrocarbon recovery from underground reservoirs, hydrocarbon wells are becoming of increasingly greater depths and more sophisticated. For example, wells exceeding 25,000 feet in depth which are highly deviated are becoming increasingly common.
Furthermore, in recognition of the expenses involved in completing and operating such hydrocarbon wells, added emphasis has been placed on well access, monitoring and management throughout its productive life. Ready access to well information and intervention may play critical roles in maximizing the life of the well and total hydrocarbon recovery. As a result, downhole tools are frequently deployed within a given hydrocarbon well throughout its life. These tools may include logging tools to acquire data relative to well conditions, intervention tools to address downhole conditions, and even downhole conveyance mechanisms such as downhole tractors to aid in achieving access to downhole portions of the well which may otherwise be potentially inaccessible.
The above noted downhole tools may be delivered to a downhole location by way of a cable. Given the depth of the well, the cable is of a configuration intended to support its own load as well as that of a toolstring of various downhole equipment. Thus, with ever increasing well depths in use, the break strength of today's cables are also increasing. Unfortunately, however, there is a limit to the benefit available from increasing the cable strength. That is, as a practical matter, an increase in the break strength of the cable also increases its overall weight, thereby adding to the load imparted on the cable. Thus, significant increases in break strength may be self-defeating. As a result, cables exceeding about 30,000 feet or so for corresponding well depths are generally impractical.
In addition to physical delivery capabilities, the cable may be configured to provide power and communication between the tool and other equipment at the surface of the oilfield. Generally, this may be achieved over a copper core or other suitable power and telemetry structure as described below. Similar to the load bearing capacity of the cable as noted above, the cable is also configured in light of these telemetry requirements and downhole power needs, especially in light of the potentially extensive length of the cable into the well.
With respect to communication over the cable, a conventional core may display about 1 dB of signal loss per every thousand feet of cable. Nevertheless, telemetry between the equipment at the surface of the oilfield and the downhole tool may remain effective over a conventional cable up until about 30 dB of signal loss has occurred. Unfortunately, this means that telemetry between the surface equipment and the downhole tool is significantly compromised over a conventional cable that exceeds about 30,000 feet. Furthermore, in circumstances where communication involves the return of signal back to the surface equipment, the return signal is even weaker upon return over such an extensive cable. In theory, the effects of such signal loss may be combated by use of a lower gauge core, say less than about 15 gauge copper wire. Unfortunately, this leads to an increase in cable profile and, perhaps more significantly, adds to the overall weight of the cable, thus further compounding load issues as described above.
As indicated, power is often provided to the downhole tool over the cable as well. For example, where a downhole tractor is present, up to 2 kW or more may be provided to the tractor over the cable. In such a circumstance, voltage and current for the power delivery may be directed at the surface. However, the particular properties of the cable may determine the particular power delivery which actually reaches the downhole tractor. For example, the loop resistance over the length of the cable may be cumulative such that power delivery is significantly affected where over about 30,000 feet of cable is employed before a downhole tool such as the tractor is reached.
For a variety of reasons as noted above, the use of downhole cables exceeding 30,000 feet is generally considered impractical for hydrocarbon well applications. Whether a matter of load, telemetry, or power limitations, cables substantially exceeding 30,000 feet or so generally remain unavailable and impractical, thereby limiting the effective monitoring and operating of wells exceeding such depths.